PAIRED FINS 127 



The phosphorescence of Elasmobranchs, mentioned by Aristotle, 

 appears to be due to special little organs scattered over the skin 

 (Burckhardt [71], Johann [246]). 



The median fins are always subdivided; the caudal is heterocercal 

 in internal structure, though the axis is almost straight in many 

 living sharks. 



The endoskeleton of the paired fins is very variable in detail, 

 owing to concentration and fusion of the radials (somactidia). A 

 large number of segments contribute to their formation generally 

 ten or more. The pectoral fins have a well-marked outstanding 

 muscular lobe, supported by a rhipidostichous skeleton (p. 106). 



Gegenbaur, in 1865, attempted to show, in an important work 



cer 



tel 



Longitudinal section of the brain of AcaiUhias. (After Johnston.) cer, cerebellum ; ep, 

 epiphysis ; h, nucleus habenulae ; l.i, lobus inferior ; l.l.l, lobus lineae lateralis ; l.v, lobus 

 visceralis ; m, medulla ; o.c, optic chiasma ; par, paraphysis ; r, roof of fourth ventricle ; r.n, 

 recessus neuroporicus ; r.p, reeessus i>raeopticu.s ; sp, spinal cord ; s.v, saccus vasculosus ; t.a, 

 tuberculuin acusticum ; tel, telencephalopi ; t.m, tectura mesencephali ; v, velum Iransversum. 



[153], that the ground-plan of the pectoral-fin skeleton consisted of 

 three basal pieces, the pro-, meso-, and metapterygium, articulated 

 to the girdle, and each bearing a number of radials. Later, he 

 compared this skeleton to the ' archipterygium ' of Ceratodus [157]. 

 The pro- and mesopterygium were considered to be formed by 

 the fusion of the basal joints of the preaxial radials. The meta- 

 pterygium, on the contrary, with sometimes some distal elements, 

 was supposed to represent the original axis. Vestiges of postaxial 

 rays are occasionally found (Fig. 53). The metapterygium was held 

 by Gegenbaur not to be formed by concrescence. 



This distinction, drawn between the posterior and largest basal 

 (the metapterygium) and the others, does not seem to be justified 

 either by comparative anatomy or by embryology. Indeed, Huxley 

 [230] identified the original axis in the mesopterygium. All the 

 basals are probably formed by concrescence, and an endless variety 



